Figuring Out Celsius-Fahrenheit

Changing Fahrenheit to Celsius by adding 40, multiplying by 5/9, and subtracting 4 degrees is great for scientists, but for the common man touring Europe it is not much help.

Here is a less accurate, but so much easier method if you are sweating and want to know in a casual way just how hot it is: Double the Celsius and add 30. Anyone can do that in his head. For 10 degrees Celsius, it gives the exact answer. For 30 degrees Celsius, it is off 4 degrees. At that temperature, who can tell the difference?